In the speed evaluation in an electronic diesel control (EDC) for diesel engines, it is known to use the so-called "segment" method in which engine speed is calculated by measuring the time interval between the passing of two successive marks (reference marks) on the engine crankshaft, these marks being equally angularly spaced. Four marks are provided for a four cylinder engine. During idling, the engine speed tends to fluctuate widely at a relatively low frequency, e.g. 5 Hz, due to the resilient mounting of the engine on the vehicle body or chassis. It is an object of the EDC to damp or suppress these low frequency speed fluctuations but superimposed on these low frequency fluctuations is a higher frequency fluctuation, e.g. 25 Hz for a four stroke four cylinder engine rotating at 750 r.p.m. This is due to the fluctuating torque produced by the combustion pressures developed in the individual engine cylinders. Using the known method, the output speed signals are almost in phase with the perfect 5 Hz waveform from which the 25 Hz frequency has been eliminated. However, the output speed signals still possess the 25 Hz component which is disadvantageous for the EDC, inasmuch as it is impossible for the EDC to compensate for the speed fluctuations due to the individual momentary combustion pressures. The 25 Hz component can be effectively eliminated by taking averages of the input speed signals but this leads to a phase delay between the perfect 5 Hz waveform and the output waveform, because the average pertaining to one reference mark cannot be calculated until a subsequent reference mark has passed.
EP-A-0 133 426 describes a method of obtaining an average value of an almost periodically fluctuating signal, such as the angular velocity of the crankshaft of a diesel engine. To obtain a tone average, it is necessary to take the coverage, such as a running average, over one or several complete periods of fluctuation. However, when the engine speed changes suddenly, a corresponding change is not reflected in a corresponding sudden change in the average value as taken but only in a gradual change. To avoid this, EP-A-0 133 426 proposes to store the angular velocity signal at discrete points and to process the stored signal values later, e.g., by comparing the stored signal values with later actual signal values. Such comparison, in the event of a change in engine speed, results in a transient deviation. A running average is also taken of the fluctuating signal representing the angular velocity of the crankshaft and any transient deviation is added to this running average to obtain an output value which accurately represents the average engine speed at any given time. Disadvantages of the method of EP-A-0 133 426 are that it requires a large storage capacity and maybe additional memory components, special measures, with resulting expense, must be taken to ensure that the stored angular velocity values and the actual values with which they are compared are in synchronism with one another, requiring additional components, and the comparison of the stored and actual values representing the course of the fluctuating signal demands large computer capacity and the large computation times offset the advantage of the method in that a rapid engine speed response is obtained.